The present invention generally relates to an image forming apparatus having an electrophotographic copying process and relates to an electrostatic latent image developing apparatus for visualizing an electrostatic latent image formed on the surface of an electrostatic latent image support by supply of developer.
Conventionally, as examples of a developing apparatus, there have been proposed arrangements shown in FIG. 12 as disclosed in Japanese Patent Application Nos. 62-47930 and 62-32047, as shown in FIG. 12. The apparatus 1' has a developing unit 3 at the front of a casing 2 and a toner hopper 4 at the back of the casing 2.
In the developing unit 3, a developing sleeve 5 having a magnetic roller 6 therein is arranged to confront a photosensitive drum 100.
In the toner hopper 4, a toner supplying roller 7 has plural minute irregularities on the outer circumferential portion thereof and is arranged to confront the developing sleeve 5. The agitating members 8 and 9 are accommodated at the rear side of the toner supplying roller 7 in the toner hopper 4. A regulating blade 10 and a toner returning prevention member 11 respectively attached to the casing 2 are contacted under pressure with the toner supplying roller 7 in the upper and lower directions thereof, respectively. The connection between the toner hopper 4 and the developing unit 3 are severed by the toner supplying roller 7, the regulating blade 10 and the toner returning prevention member 11.
According to the above-mentioned developing apparatus 1', toner accommodated in the toner hopper 4 is transferred forwardly on the basis of the rotation of the agitating members 8 and 9. Then, toner positioned at the vicinity of the outer circumferential portion of the toner supplying roller 7 is transferred in a direction as indicated by an arrow while the toner is held in the minute irregularities of the surface of the toner supplying roller 7. Then, part of the toner held on the surface of the toner supplying roller 7 is transferred through the vicinity of the distal end contact portion of the regulating blade 10 to a region confronting the developing sleeve 5. The reminder of the toner is regulated by the distal end portion of the regulating blade 10 to be scrapped off from the surface of the toner supplying roller 7 and is collected in the toner hopper 4.
Toner transferred to the region confronting the developing sleeve 5 is supplied to the surface of the developing sleeve 5 by scraping action of carrier magnetically held on the outer circumferential portion of the developing sleeve 5 and by an electric suction force based on a voltage difference between a developing bias voltage Vb applied to the developing sleeve 6 and a returning bias voltage of alternating current biased a D.C. voltage Vss and an A.C. voltage Vrms respectively applied to the toner supplying roller 7.
Toner supplied to the developing sleeve 5 is mixed with the carrier at the outer circumferential portion of the developing sleeve 5. Thus, developer consisting of mixture of mixed toner with the carrier at a specified rate is prepared and is transferred to a region confronting the photosensitive drum 100 to visualize an electrostatic latent image formed on the surface of the photosensitive drum 100.
In the arrangement of the developing apparatus 1', however, as mentioned above, the distal end of the regulating blade 10 contacts with the outer circumferential portion of the toner supplying roller 7 under pressure, on which the minute irregularities is formed. Thus, excessive toner is scrapped off by the blade 10 to regulate the amount of toner supplied to the developing unit 3.
Accordingly, an average grain diameter of toner supplied to the developing unit 3 relates to the roughness of the minute irregularities formed on the surface of the toner supplying roller 7 and the average grain diameter of toner to be used. But, since there is generally a high probability that toner having small grain diameter is held in the minute irregularities and toner having big grain diameter is scrapped off by the regulating blade 10, toner having small grain diameters has a preference to be consumed. That is, grain diameter of toner to be used is sorted.
Then, when the toner is replenished the toner hopper 4, a preference is firstly given to toner having small grain diameter to be supplied to form a minute image. After that, when toner having big grain diameter becomes to be supplied in accordance with increasing the number of sheets of the paper to be copied, the following problems arise. That is, according to increasing grain diameter of toner, the bigger toner to be supplied becomes, the more the charging property of toner falls. Thus, texture of the image becomes rougher and is caused on the copied paper so that the quality of the image on the copied paper becomes bad.
Hereinbelow, a result of a concrete experiment about the sorting of grain diameter of toner is described.
The experiment was performed as follows: toner having average grain diameter of about 14 .mu.m was inserted into the toner hopper of the developing apparatus 1' as shown in FIG. 12. The grain diameter of toner was measured after each specified number of the copied paper at a measured point B on the surface of the photosensitive drum 100 after passing through the region confronting the developing sleeve 6, a measured point C in the toner hopper 4, and a measured point D on the surface of the toner supplying roller 7 after passing through the vicinity of the distal end of the regulating blade 10, respectively.
The results of the experiment is shown in FIG. 13. As is clear from the drawing in FIG. 13, though the value of the toner average grain diameter measured at the measured point C in the toner hopper 4 was firstly about 14 .mu.m, the value increased in accordance with increasing the number of sheets of the copied paper. And thus the value reached about 16 .mu.m in about 1,000 sheets of the copied papers and about 20 .mu.m in about 2,000 sheets of the copied papers.
The value of the toner average grain diameter measured at the measured point D after passing through the vicinity of the distal end of the regulating blade 10, and the value of the toner average grain diameter measured at the measured point B on the surface of the photosensitive drum 100 were respectively about 12 .mu.m and about 12.5 .mu.m, fewer less than 14 .mu.m which is the value of the average grain diameter of toner firstly inserted into the toner hopper. But, the values increased in accordance with increasing the grain diameter of toner in the toner hopper 4, and thus were respectively about 13.2 .mu.m and about 13.6 .mu.m for 1,000 sheets of the copied papers. Then, the values were respectively about 16.4 .mu.m and about 17 .mu.m for 2,000 sheets of the copied papers, much more than that of average grain diameter of toner firstly inserted into the toner hopper.
Meanwhile, though the qualities of the images on the copied papers were good in 1,000 sheets of the copied papers, the qualities thereof became rougher in 2,000 sheets of the copied papers and fog was found on the copied papers additionally.
Although not shown in FIG. 13, after the number of sheets of the copied papers reached 2,000, new toner having average grain diameter of 14 .mu.m was replenished in the toner hopper 4. After that, a lot of fog was found on several copied papers.